This invention relates to an acoustic flowmeter and more particularly to such a flowmeter having improved accuracy, sensitivity and response time, achieved by narrow band signal processing.
Various systems have been developed for measuring the velocity of a fluid flow by utilizing the effect the fluid velocity will have on the transit time of an acoustic pulse transmitted along the direction of flow. One example of such a system uses a dual "sing-around" technique in which one series of pulses is transmitted in one direction, e.g. upstream, and another series of pulses is transmitted in the opposite direction, e.g. downstream. When each pulse in a series is received, it causes the next pulse to be transmitted in the same direction. Accordingly, the repetition rate of the pulses in each direction is predominantly determined by the transit time through the fluid medium. Since the transit time is affected by the flow velocity, the difference in frequency between the two series of pulses will reflect a measurement of fluid velocity.
The principal difficulty with this "sing-around" type of system is that the frequency difference will typically be very small as compared with the frequency itself and thus each of the sing-around oscillators must be stable to a very high degree of precision. Further, since the transmit signals are in the nature of pulses, broadband transducers and electronics are required which increase the difficulty of obtaining a satisfactory signal-to-noise ratio.
Further, since each of the oscillators must function in the nature of a phase lock loop, the presence of noise or turbulence in the fluid may cause a loss of the phase lock and a period during which the instrument is insensitive and there may even be a failure to recapture loop lock.
Among the several objects of the present invention may be noted the provision of an acoustic flowmeter which is capable of a high degree of precision; the provision of such a flowmeter which utilizes narrow band transducers and signal processing electronics; the provision of such a flowmeter which has a relatively short response time with respect to changes in fluid velocity; the provision of such a flowmeter which has an advantageous signal-to-noise ratio characteristic; the provision of such a flowmeter which is reliable and which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.